Binary multiplier employing multiple input threshold gate adders



Aug. 18, 1970 MAQ c. WANG 3,524,977

BINARY MULTIPLIER EMPLOYING MULTIPL INPUT THRESHOLD GATE ADDERS Filed Jan. 17, 1967 7 Sheets-sheet 1 '2 z f Jaz 14o/*4f y ,Y` .d

/aL/J/ z'fzzf/ MM' I IW afl l -v 'di y 4r Mr,

l I Wf/J z f J f ,/aa fdff) f4 f f z Z f//Z 3,524,977 BINARY MULTIPLIER E'MPLOYING MULTIPL INPUT THREsHoLD GATE ADDERs Filed Jan. 17, 1967 Aug.` 18, 1970 MAQ c. WANG 7 Shets-Sheet 2 i fffzzzz 74A/PUT nope/z f4, :1; /5 auf fum/V51 f-lA/PUT #DDEE 4l/7.1.4: {ll- 411111 Il Aug. 18, 1970 .BINARY MULTIPLIEIR EMPLOYING MULTIPLEA INPUT THRESHOLD GATE ADDERS Filed Jan. 17,

MAO C. WANG 7 Sheets-Sheet B Aug.V 184, 1970 MAC)l C. WANG BINARY MULTIPLIER EMPLOYING'MULTIPLE INPUT THRESHOLD c-TB ADDERS Filed Jan. 17, 1967 7 Sheets-Sheet 4 /f/f/ for Aug.y l8v, 1,970 I MAo'c. WANGy l 3,524,977

BIARY MULTIPLIER EMPILOYING MULTIPLE INPUT` THRESHOLD GATE ADDERS -BINARY MULTIPLIER EMPLOYING MULTIPLE INPUT THRESHOLD GATE ADDERS Filed Jan.y 17, 1967 7 Sheets-.Sheet 6 A33 mswuo .o2 mmt wmwwunm N. gl summe. zt QMQSRQQ 7 Sheets-Sheet '7 Aug., 18, 119470v `MAO c,.wANG

BINARY MULTIPLIER. EMBLOYING MULTIPLE INPUT `THRIJSHOLDm13 ADnERs Filed Jan, 17, 196,7

v K `\wm.\lg\\9"`9` V www l Q C ,W .1. .www S LQQ Sm lJ. m n Lsung, um I ,NCJ, ma@ .0 zoQQQH EMM wwww, w E ,E ,4H g5. t@ .5 @v U :SSS 2 E N United States Patent O 3,524,977 BINARY MULTIPLIER EMPLOYING MULTIPLE INPUT THRESHOLD GATE ADDERS Mao C. Wang, Westmont, NJ., assignor to RCA Corporation, a corporation of Delaware Filed Jan. 17, 1967, Ser. No. 609,949 Int. Cl. G0611' 7/52 U.S. Cl. 23S-164 1 Claim ABSTRACT F THE DISCLOSURE This disclosure relates to binary arithmetic circuits and particularly to multipliers. Some important features of these multipliers are their exceptionally high speed and their capability of being implemented with threshold gates. These multipliers employ multiple input (2, 3 and higher numbers of input) adders.

BACKGROUND OF THE INVENTION Binary multiplication in Idigital computers normally involves multiplying the multiplicand or stored multiple thereof by a multiplier digit to obtain a partial product andl then temporarily storing the same; following the same procedure for the next most significant multiplier digit to obtain a second partial product; relatively shifting the second partial product and adding it to the previously obtained stored partial product to obtain a third partial product; and continuing a similar process until the remaining multiplier digits have been exhausted and the nal product obtained. This type of multiplication is relatively time consuming and requires the usual equipment in a general purpose computerstorage registers, gates, arithmetic unit and the like, operated under the control of stored program instructions.

SUMMARY OF THE INVENTION The multiplier of the present invention is a special purpose computer which includes means for concurrently obtaining the partial product of each multiplier bit and the multiplicand and a plurality of multiple bit adder means for concurrently performing additions. Each adder' means produces an output (a nal product bit) indicative of the sum of all partial product bits of a given signiicance and of the carry bit or bits produced in obtaining the final product bit of next lower significance. This multiplier operates at veryhigh speed. For example, in an S-bit x 8-bit multiplier discussed below, only l5 stage delays are needed to obtain even the slowest arriving final product bit.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram of an AND gate employed in the networks of the invention;

FIGS. 2-10 are block circuit diagrams of multiple input adders which may be employed in the multipliers of the invention, each adder being appropriately labelled;

FIG. 11 is a block circuit diagram of a 3-bit by 3-bit multiplier according to the invention;

FIGS. 12a, 12b and 12e together show an 8-bit by 8-bit multiplier according to the invention; and

FIG. 13 is a chart to help explain the operation of the adder of FIGS. 12a-12C.

DETAILED DESCRIPTION The multipliers of the invention are new and improved networks which are implemented in a way which is in itself new. The multipliers are preferably made up entirely of threshold gate circuits. Before discussing the networks, the threshold gate circuits themselves are discussed.

FIG. 1 shows a threshold gate implementation of an AND gate. It consists of a 3input threshold gate, each input having the weight 1 and the gate having a threshold of 2. This particular gate is knwn as a 3-input majority gate.

The three input terminals of the circuit above are adapted to receive three electrical signals and the circuit produces an electrical signal at its output. These signals represent binary digits and for the sake of brevity in the discussion of this gate and other circuits which follow, the binary digits or bits themselves will be referred to rather than the signals manifesting the bits.

In the operation of the AND gate of FIG. 1, if both x and y represent a l, z will represent a 1 because the majority of the inputs are l. If one or both of x and y is a yD', then two or three of the inputs to the gate are 0 and the output z is 0.

The networks of FIGS. 2 through 10 are all binary a-dders. In .the case of the 2 and 3 input adders there is one sum bit S and one carry bit W which is produced. The carry bit W is produced on the a lead. In the case of the 4 and 5 bit adders, there is one sum bit and two carry bits which are produced. The latter are produced on the a and b wires, respectively. In the case of the 6 and 7 bit adders of FIGS. 9 and 10, there is one sum bit and three carry bits which are produced. The latter areV TABLE I No. of Input ls Hendon-Ionio v-n-u-u-u-u-oo i-u-udi-loooo hu-loooooo The 2-input adder of FIG. 2 consists of three stages 100, 101 and 102. Stage 100 is the AND gate of FIG. l. Stage 101 has a total of 7 input weights and a threshold of 4. The weights are represented by numerals within the block 101. Threshold gate 102 has 5 input leads, a threshold of 5 and input weights 3, 2, 2, 1, 1, a total of 9. (Weighted threshold gates are in themselves known and are described in greater detail in application Ser. No. 547,943, by Robert O. Winder, filed May 5, 1966, assigned to the same assignee as the present application.)

There are a total of 4 variables x1, y0, x0, y1 which are applied to the adder of FIG. 2. Nevertheless, this adder is termed a 2-input adder because it obtains the sum of Ao and A1 where A0=x1y0 and A1|=x0y1. A logical product such as A1 can be considered to be present within the adder.

It readily can be shown that the adder of FIG. 2 does operate in accordance with Table I. For example, assume that A1 and An are both O. In this case at least 5 (suppose x0=x1=0 and y0'=y1\=l) of the 7-input weights to gate 101 are 0 so that W, the carry, equals 0. As 146:0, gate produces a 0 output. Therefore, at least 5 of the 9 input weights to gate 102 represent 0 and the sum S=0.

Assume A1=l and A40:0. At least 4 of the 7 input weights to gate 101 are 0 so that W is still 0. However, the complementary or minority output of gate 101, indicated by the half-circle 104, is a l. A1 is also a 1 so that 5 of the 9 input weights to gate 102 are 1. Therefore, the sum produced S=l.

The last case which need be considered for FIG. 2 is A1 and A0 both equal to 1. Now, 4 of the 7 input weights to gate 101 are 1 so that W=1. 5 of the 9 input weights to gate 102 are 0 so that S=0.

The 3input adder of FIG. 3 consists of two threshold gates 106 and 108, the first a simple 3input majority gate and the second a 4-input gate having a threshold of 3 and input weights 2, 1, 1, 1. Again, the operation of this gate can be explained by a few examples. Frst, assume that the input carry W1 and the input bits A0 and A1 are all 1. In this case, the output carry W1+1 is 1. Similarly, since 3 of the 5 inputs to gate 108 are 1, S1 is 1. Note that this agrees with the table. As a second example, assume that A is 1 and the other two inputs are a 0. The output carry W is a 0 since two of the three inputs to gate 106 are 0. The output sum S1 is l because three of the input weights to gate 108 are 1. (The complementary output of gate 106 is a 1 and it has the weight 2 and A0 is a 1 also so that the total number of input weights is three, as stated.)

The 3input adder of FIG. 4 consists of 2 threshold gates 110 and 112. Gate 110 has 5 input weights, 1, l, 2, 2, l, respectively and has a threshold of 4; gate 112 has 6 inputs, a threshold of 6, and input weights l, 4, l, 1, 2, 2, respectively. As in the case of the 2-input adder of FIG. 2 the adder of FIG. 4 is termed a 3-input adder because the adder considers the inputs x and y to be a logical product A0. The 3-inputs therefore are assumed to be A0, A1 and W1. The operation of this circuit should be clear from the explanation given of the circuit of FIG. 3. However, one example will be given for purposes of illustration. Assume x and y are both 0 and A1 and Wi are both l. Four of the 7 input weights to gate 110 are l and this gate produces an output carry W=l. Seven of the 11 input weights to gate 12 are 0 so that the sum output S1 IS 0.

The 4input adder of FIG. 5 consists of three threshold gates 116, 118 and 120. Gate 116 is a 5-input gate with a threshold of 4 and the respective inputs are weighted 3, 1, 1, 1, l. Gate 118 is a S-input gate with a threshold of 3 and with all inputs of weight 1. Gate 120 is a 7input gate with a threshold of 5 and with input weights 2, 2, l, l, l, l, l, respectively. The operation of the circuit of FIG. 5 may be explained Iby way of a few examples. Assume that all four inputs are 1. In this case, 4 of the input weights to gate 116 are l and 5 of the input weights to gate 118 are 1 so that the two carries produced are both 1. The complementary outputs of gates 116 and 118 are both 0. Therefore ve of the nine input weights to gate 120 are 0 and the sum output is S=0. Assume now that three of the four inputs are 1. At gate 116 only three of the 7 input Weights are l so that the gate produces an output carry which is 0 at its b output lead. Four of the 5 input weights to gate 118 are 1 so that this gate produces an output carry of 1 at its a output lead. Five of the 9 input weights to gate 120 are 1 so that the sum output of this gate is a l.

The 4-input adder of FIG. 6 consists of three gates 122, 124 and 126. Gate 122 is a 6-input gate having a threshold of 8 and input weights of 7, l, 1, 2, 2, 2, respectively; gate 124 is a 6-input gate with a threshold of 5 and input weights of 2, 2, 2, 1, 1, 1, respectively; gate 126 is an 8- input gate having a threshold of l0` and input weights of 4, 4, 2, 2, 2, 1, 1, 3, respectively.

The operation of the adder of FIG. 6 is analogous to that of the one of FIG. 5. Note that in this adder as in some of the others, the fourth input is considered to be A0, the logical product of x and y.

The 5-input adder of FIG. 7 comprises three gates 128, 130 and 132. Gates 128 and 130 are `6-input gates with a threshold of 4 and with input weights 2, 1, 1, 1, 1, 1. Gate 132 is a 7input gate having a threshold of 5 and with input weights 2, 2, 1, 1, 1, 1, 1, respectively.

In the operation of the adder of FIG. 7, assume that one of the S-input bits has the value 1 and the remaining bits are all 0. Gate 128 produces an output carry of 0. Gate 130 produces an output carry of 0 as four of the 7 input weights are 0. Gate 132 produces an output S=1 as 5 of the 9 input weights are 1.

As a second example, assume that all 5 input bits are 1. Gates 128 and 130 both produce an output carry equal to 1. Gate 132 produces a sum output equal to 1 as five of the 9 input weights are l.

The S-input adder of FIG. 8 comprises three gates 134, 136 and 138. Gate 134 is a 7input gate with a threshold of 8 and with input weights 5, l, 1, 2, 2, 2, 2; gate 136 is a 7input gate with a threshold of 7 and with input weights 3, 2, 2, 2, 2, 1, 1; gate 138 is a 9-input gate with a threshold of 10 and with input weights 4, 4, 2, 2, 2, 2, 1, 1, 1.

The operation of the circuit of FIG. 8 is analogous to that of the circuit of FIG. 7. Again the fifth input is considered to be AD which is the logical product of x and y.

The 6-input adder of FIG. 9 comprises four threshold gates 140, 142, 144 and 146. Gate 140 is a 7input gate having a threshold of 6 and input weights of 5, l, 1, 1, l, l, 1; gate 142 is a simple 7input majority gate which has a threshold of 4 and in which each input has the weight 1; gate 144 is a 7input gate with a threshold of 5 and input weights 3, l, l, 1, l, l, l; gate 146 is a l0-input threshold gate with a threshold of 7 and with input weights 2, 2, 2, l, l,l,1,l,l,l.

In the operation of the adder of FIG. 9, assume all of the inputs have the value 0. Gate 140 produces a carry =0 output since all eleven input weights are 0. Gate 142 produces an output carry equal to 0 since all seven input weights are 0. Gate 144 produces an output carry equal to 0 since six of the 9 input weights are 0. Gate 146 produces a sum :0 output as seven of the 13 input weights are 0.

As a second example, assume that three of the inputs are 0 and three are 1. Eight of the l1 input weights to gate 140 are 0 so that the output carry it produces is O. Four of the input weights to gate 142 are 0 so the output carry it produces is 0. Six of the 9 input weights to gate 144 are 1 so that it produces an output carry equal to 1. Seven of the 13 input weights to gate 146 are 1 so that it produces a sum output equal to l.

As a third example, assume that all six input bits are a 1. Gates 140, 142 and 144 all produce output carries equal to l as should be clear from inspection. Gate 146, however, has 7 of its 13 input weights equal to 0 and therefore produces an output sum equal to 0. Note that all of these examples agree with Table I.

The 7input adder of FIG. 10 comprises four threshold gates 148, 150, 152 and 154. Gate 148 has 8 inputs, a threshold of 6 and input weights of 4, l, l, 1, l, l, l, 1; gate is a simple majority gate with a threshold of 4 and with 7 inputs all of weight l; gate 152 is the same. as gate 148, however, it has a fixed bias of l rather than 0 applied to input terminal of weight 4; gate 154 has l0 inputs, a threshold of 7 and input weights of 2, 2, 2, 1, 1, 1, 1, 1, l, l. The operation of the gate should be clear from the discussion of the previous adders. The gate does implement the sum function as shown in Table I.

A 3 x 3 multiplier according to the invention is shown in FIG. 11. It operates in the way shown in Table II In the operation depicted in Table II, the circuit obtains the logical product of x1 and y1 and this is the product bit of least significance S1. The circuit obtains the logical product of x1 and yz and x2 and y1, and adds these two quantities directly to obtain the product bit of next significance S2 and a carry bit W3. In Table II the solid line rectangle around W3 indicates that it is a carry.

The circuit adds the logical products y3x1, y3x3, y1x3 to the carry bit W3 above to obtain the final product bit of next significance S3. During the addition, two carries W4 and W43 may be generated.

The remainder of the operation should be clear from the chart.

In the chart of Table II, the assumption is made that the least signicant bit is y1. Note that the first row of the chart is the product of the least significant multiplier bit x1 and the multiplicand y3, y2, y1. The second row represents the product of the next significant multiplier bit x2 and the multiplicand, and so on. Note that the second and third pratial products are each shifted one place to the left. As will be shown shortly, this shifting is automatic.

FIG. 11 should now be referred to. AND gate 156 produces the least significant product bit S1. Gates 158, 160 and 162 together comprise a 2input adder. They obtain the sum of yzxl and ylxz. This sum S3 is the next most significant bit of the product as should be clear from Table II. The carry W3 generated by stage 160 is applied to the following adder.

Stages 164 and 166 together comprise a 3input adder auch as shown in FIG. 4. This adder obtains the sum of W3, the previous carry, and x2y2 and x3y1. The sum SP3 produced at stage 166 is a partial product bit. The carry W4 propagates to the left to tht next adder stage.

The stages 168 and 170 together comprise a 2input adder of the type shown in FIG. 2. They add the partial product bit SP3 to the logical product y3x1. The sum bit produced, that is S3, is the next most significant product bit. The carry bit W4,1 propagates to the left to the next adder.

The stages 172 and 174 together comprise a 2input adder which adds the carry W4 to the logical product x3y3. The sum bit SP4 which is generated is a partial product bit. The carry bit W5 is employed to calculate the most significant product bit S3 and also the product bit S5.

The next two stages 176 and 178 together comprise a 3input adder. They add the partial product bit SP4, the carry bit W43 and the logical sum x3y2 together. The sum bit obtained S4 is the final product bit of next significance. 'I'he carry bit W53 which is generated is employed in obtaining the two most significant product bits.

The final stages 180 and 182 together comprise a 3- input adder. They add the two carries W5 and W5, to x3y3 to obtain a sum bit S5 which is the next to the most significant final product bit and a carry bit S3 which is the most significant product bit. I

An important advantage of the circuit of FIG. 1l is its very high speed. No storage or shifting is necessary. All adders produce concurrent outputs, that is, each sum and carry signal is present during a common time interval, although the leading edges of many of these signals are delayed relative to one another by intervals proportional to the number of stages employed to produce such signals. The bit of the final product delayed the greatest interval of time (the worst case) is obtained with seven stage delays.

While the network of FIG. ll is shown to be implemented solely with threshold gates, by way of example, the invention is also adaptable to the use of other, non-threshold gate circuits which can perform the same logical functions. The network of FIG. ll may be designed, for example, with NOR gates. Such a design can be shown to require 41 gates, to have a total of 87 gate leads and to have eight stage delays. The same circuit implemented with threshold gates, as shown, employs only 15 gates, has a total of only 70' gate terminals and has only seven stage delays.

The design of an 8 x 8 binary multiplier is shown in FIG. 13. The least significant input bits are x0 and y0,

respectively and most significant are xr, and yq. A bit such be represented by because the logical product need not be obtained separately.

A square around a letter indicates that the bit represented 'by the letter is obtained as a carry output of an adder. A dashed circle around a letter indicates that this bit is obtained as a sum output of an adder. For example, in the addition of to Di to Dz shown in column 5,

asumen?, D

are obtained. In the addition of the bits of column 3, that is, the addition of and a carry bit E? B0, B1 and B2 and B31 the sum bn s 3 :land the carry bits Ci and C5 are obtained.

The rows of the table of FIG. 13 represent logical` products. For example, the rst row G0, F3, E0, D3, C0, B0, A3, S0 is the logical product of the multiplicand x7 x0 and the least significant multiplier bit yo.

The superscripts in the table indicate the number of stage delays. The subscripts applied to the final product bits indicate the signicance of these bits. For example, S3 is the least significant sum bit and, since it has no superscript, it is obtained in one stage delay. The

product bit Si (column 2, row 3) is the 21 product bit and it is obtained in two stage delays. The

sum bit (column 12, row l5) is the 211 product bit and it is obtained in l2 stage delays.

The table also shows that the final product bits are obtained by adding together all of the bits appearing in that same column. For example, the

sum bit S2 is obtained in the following Way. The -bits D3, D1 and D3 are added in a 3-bit adder. The

partial product bit D2 (the sum output of the adder) thereby obtained is added t0 D3 and D4 and D?, and D? and D2 in a 6-bit adder. Note that of these six bits, the last three mentioned are carry bits generated during the operation deplcted 1n column 4. The sum obtained as a result of this 6bit addition is the 24 product bit S2 and it is obtained in five stage delays.

The multiplier shown symbolically in FIG. 13 is shown 1n block circuit diagram form in FIGS. 12a, 12b and 12C.

The adders of these gures are the threshold gate adders of FIGS. 2-10. From the foregoing explanation, the op-` eration of the multiplier of FIGS. 12a-12C should be easy to follow. For example, the least significant product bit which is the logical product of x and y0, S0 is obtained by an AND gate 1. The 21 product 'bit is produced by the 2-input adder 2 (the details of which are given in FIG. 2). The latter adds A0 and A1, that is, it adds xlyo and xoyl. The 2-input adder 2 also produces a carry B which in FIG. 13 is shown to propagate to the next column to the'left and in FIG. 12a is shown as on e of the inputs to the 4-input adder 7 (the details of which are given in FIG. 5). Returning again to FIG. 13, it is seen that the 22 product bit S is obtained by adding B0, B1, B2 and the carry bit B The three AND gates 4, 5 and 6 of FIG. 12a obtain the B bits B0, B1 and B2 and the 4-input adder 7 adds these three bits to the carry bit Bl to obtain the 22 product bit S2 As one final example, it may be observed from FIG. 13, column 10, that the bits I0 I5 and I must be added together to obtain a partial product bit (the sum bit) I and three carry bits J?, J? J2 The stage which implements this function is shown at the lower right of FIG. 12b. It is a 7-input adder 62 (the details of which are given in FIG. which receives the I bits generated by AND gates 56 through 61 and which receives also the carry bit I produced 'by a previous adder 49 shown at the upper left of FIG. 12b. The adder 62 produces the partial product bit I and three carry bits J2 J? J2 While the invention has been illustrated in terms of a 3-bit `x 3-bit multiplier and an S-bit x 8bit multipler it is to be appreciated that the principles are applicable to multipliers of any size. It should also be appreciated that the partial product bits may be added in any convenient way. Thus, if it is found that the fan-in requirement (the number of inputs needed to a logic stage) is too severe in a particular engineering design, 6-input and 7-input adders need not be employed and the'entire adder designed instead with 2, 3, 4- and 5-input adders or even with only 2- and 3input adders. (As an example, rather than employing a 6-bit adder to add the 6 C bits of column 4 of FIG. 13, two 3-bit adders and one 2-bit adder may be used instead. The rst 3-bit adder adds Co-i- 01C:

the second adds the sum bit produced to C3 and C2 the 2-bit adder adds the sum bit produced by the second adder to Ci) The penalty generally paid, however, when smaller numbers of input adders are employed, is an increase in the number of stage delays. On the other hand, it is possible to use 8, 9, l0 or higher numbers of input adders. The use of the larger number of input adders reduces the number of stage delays and the total number of gates but the engineering tolerances become more severe.

The use of threshold logic stages to implement the multipliers of the invention results in a great saving in equipment. In the multiplier of FIGS. 12a, 12b and 12e there is a total of 138 gates, the maximum fan-in is 13, all sum and carry bits are produced concurrently, that is, during a common time interval they are all present and the maximum number of stage delays is 15. A NOR gate implementation of an adder such as this requires about three times this number of gates (422 to be exact) and there is a substantially greater number of stage delays (a maximum of 28 in one particular design).

What is claimed is: 1. A binary multiplier for multiplying a multiplicand word yn, y 1 y0 by a multiplier word xn, x 1 xo comprising, in combination:

means for producing an output signal indicative of xoyo; a plurality of multiple bit adder means, each consisting solely of interconnected threshold gates, each for producing signals indicative of the sum and carry of a group of bits chosen from A, xy, S and W bits, where A is a partial product and consists of the logical product of an x bit and a y bit, xy is a logical product, S is a sum bit, and W is a carry bit; means for concurrently applying to each adder means a group of signals chosen from signals representing I`A, x, y, S and W bits for causing each adder means to produce output signals representing S and W bits, where the signals representing S and W bits applied to any adder means and derived from another adder means; and means for interconnecting said adder means in such a way as concurrently to produce sum signal outputs indicative of the sums of partial products of the multiplicand and multiplier bits which sum signal outputs represent the 'bits other than xoyo of the product of the multiplicand and multiplier words.

References Cited UNITED STATES PATENTS MALCOLM A. MORRISON, Primary Examiner D. H. MALZAHN, Assistant Examiner U.S. C1. X.R. 235-172 

